Hi, thank you for the reply, unfortunately all these checked out. The symptom is still when you sit at idle the engine is very rough and even at 750 revs wants to chug ahead, and after a number of seconds, stalls. One merc mechanic says that the engine is being overfuelled possibly by the cold start device causing an overrich mixture. He suggested closing off the fuel to the cold start device. Any advice please. Fastfree.

1.
Disconnect oxygen sensor plug and connect male terminal to ground. Start engine. Test sensor voltage between female terminal and ground. Sensor voltage should be near 450 mV. If not about
450 mV, replace oxygen sensor. If okay, go to next step.
2. Stop engine. Check wire from oxygen sensor plug to CIS-E control unit (terminal No. 8) for continuity. Resistance should be about zero ohms. If resistance is not about zero ohms, repair
circuit. If okay, test is complete.